In recent years, various technologies for automatically stopping the engine while the vehicle is stopped in an idling state and automatically restarting the engine so as to smoothly start the vehicle have been proposed as methods for reducing or controlling exhaust emissions and improving the fuel economy. With regard to these technologies, if it takes an undesirably long time to restart the engine, the driveability may deteriorate due to a delay in response to the driver's intention of starting, and it is therefore important to quickly restart the engine. Generally, a starter motor is used for starting the engine, which makes it difficult to quickly restart the engine. Furthermore, the above-described technologies require the engine to be frequently stopped and started in a repeated fashion, resulting in reduction of service life of the starter motor and its peripheral parts, and reduction of the amount of electric power charged in the battery due to excessive use of the battery.
In view of the above problems, an engine starting system as disclosed in, for example, Japanese Laid-open Patent Publication No. 2004-124754 may be applied to a direct in-cylinder injection type engine in which fuel is injected directly into a combustion chamber rather than an intake port. The starting system disclosed in this publication makes it possible to start this type of engine without using a starter motor. When the direct in-cylinder injection type engine is restarted, the engine starting system as disclosed in the above publication controls the throttle valve to be placed in a predetermined open position for a certain period of time during a stopping process or rundown of the engine, so as to increase the resistance to the movement of pistons toward the top dead center in a cylinder that is in the compression stroke and a cylinder that is in the expansion stroke. With the throttle valve opened in this manner, the piston of each cylinder can be stopped at a suitable stopping position, and an appropriate amount of air is surely charged into the expansion-stroke cylinder.
In the conventional engine starting system as described above, the throttle valve is controlled to be placed in the predetermined open position for a certain period of time during the rundown of the engine, so that the piston of each cylinder is stopped at a suitable stopping position, and a sufficient amount of air is contained in the expansion-stroke cylinder. However, if the throttle valve is suddenly opened while the engine is being stopped, upon meeting of engine stop conditions, from an operating condition (e.g., an idling state) in which the intake manifold vacuum is large, air rapidly flows into the intake pipe, causing a problem such as occurrence of unusual sound or noise.